Polyolefin-based thermoplastic resin compositions show excellent moldability in various molding processes, and they are used for interior or exterior parts of transporting means including automobiles and vessels and for various structural materials in various industrial fields such as household electric appliances, office machines and tools, furniture, living necessaries, miscellaneous goods, and building. A coating is often applied on molded products of these thermoplastic resin compositions for such purposes as surface protection and coloring. However, since polyolefin-based thermoplastic resin compositions are nonpolar materials having poor affinity for coatings, various pretreatments are required for the coating of these resin compositions. Generally known as such pretreatments are primer treatment, corona treatment, and the like.
The primer treatment is a technique of coating a molded product of a polyolefin-based thermoplastic resin composition with a compound, such as a chlorinated polyolefin, having an affinity for both the thermoplastic resin composition and a coating to be applied, thereby to improve the affinity of the molded product for the coating.
However, primers are expensive, and the use thereof necessitates a high-temperature and time-consuming drying step for solvent volatilization because they contain a solvent in a large amount. The primer treatment is also disadvantageous in that a pretreatment with a vapor of trichloroethane (TCE) should be conducted prior to application of a primer so as to improve the adhesion of the primer to the molded product. The primer treatment has these drawbacks and, in particular, the use of TCE is coming to be regulated in recent years because of environmental pollution problems.
The corona discharge treatment is a technique of modifying the surface of a molded product to improve the affinity of the molded product for a coating to be applied. This treatment, however, is disadvantageous in that the molded product is limited in shape and that an investment in the treating equipment is necessary.
If coatings can be directly applied to molded products of thermoplastic resin compositions without either of these treatments, not only a coating process can be shortened, but also coated products can be provided without using harmful substances such as solvents and TCE. Thus, the direct coating is very preferable in that the number of steps can be reduced and equipment investment is unnecessary.
It has been attempted to improve the coating affinity of thermoplastic resin compositions, as described in European Patent 336780. The European Patent discloses a method in which a mixture consisting of a peroxide-crosslinkable olefin copolymer rubber, an olefin-based resin, and a monomer having one or more amino groups is dynamically treated with an organic peroxide at an elevated temperature and the thus-obtained thermoplastic resin composition is blended with an unsaturated carboxylic acid derivative with heating. The European Patent also discloses a method in which a mixture consisting of a peroxide-crosslinkable olefin copolymer rubber, an olefin-based resin, and an unsaturated carboxylic acid or a derivative thereof is dynamically treated with an organic peroxide at an elevated temperature and the thus-obtained thermoplastic resin composition is blended, with heating, with a monomer having one or more amino groups. However, these techniques fail to sufficiently improves coating affinity, and satisfactory coating adhesion strength can not be attained.
A thermoplastic resin composition modified with a hydroxyl group-containing compound is disclosed, for example, in U.S. Pat. 4,908,412. This thermoplastic resin composition is prepared by a method in which a mixture of a peroxide-crosslinkable olefin copolymer rubber, an olefin-based resin, and an unsaturated epoxy group-containing monomer or an unsaturated hydroxy group-containing monomer is dynamically treated with an organic peroxide at an elevated temperature. However, this reference gives no description concerning the coating affinity of the thermoplastic resin composition.
Further, JP-A-58-191706, JP-A-60-55012, and JP-A-60-55052 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") disclose a composition comprising a polypropylene modified with an unsaturated hydroxy group-containing compound, and an ethylene-propylene copolymer rubber. However, the coating adhesion strength of this composition is not always high.
As apparent from the above, when a coating is applied directly on molded products obtained from these resin compositions, peeling of the coating film is apt to occur. It is, therefore, difficult to attain a particular purpose, such as coloring or surface protection, without a pretreatment such as those described hereinabove.
JP-A-4-272938 and JP-A-4-272948 disclose resin compositions having improved coating affinity, which comprise a blend of a polyolefin resin with (i) a hydroxyl group-terminated diene polymer or a hydrogenation product thereof, or a copolymer of ethylene and a hydroxyl group-containing unsaturated compound, (ii) an organotin compound, and (iii) a tertiary amine compound, and further with a styrene-based or olefin-based (e.g., ethylene-based) elastomer as an optional ingredient. However, the coating affinity of these compositions is still unsatisfactory.
Coating methods and coated moldings are disclosed in JP-A-3-157168, JP-A-3-157169, and European Patent 429236. In these references, nonuse of a primer is clearly stated. However, TCE treatment is performed in most of these techniques and a satisfactory coated product cannot be obtained by any of these techniques without a pretreatment.